1. Field of the Invention
The present invention relates to the field of optically pumped atomic frequency standards and other optical pumping systems, and more particularly to a method and system for suppressing or eliminating light shift in an optical pumping system by using a modulated radiation source for simultaneously locking the frequency of the radiation source to an atomic resonance and locking the frequency of the optical pumping source.
2. Description of the Related Art
Two examples of optical pumping systems are atomic frequency standards and atomic magnetometers. Atomic frequency standards, or atomic clocks, are applied in various systems that require extremely accurate frequency measurements. Atomic magnetometers are utilized in magnetic field detection with extremely high sensitivity. For example, atomic clocks are used in GPS (global positioning system) satellites and other navigation systems, as well as in high-speed digital communication systems, scientific experiments, and military applications. Magnetometers are used in medical systems, scientific experiments, industry and military applications.
Optical pumping of atomic samples in optical pumping systems is accomplished by means of one or more pumping sources, which for example can be a discharge lamp or a laser. Optical pumping systems use an atomic sample, which can comprise an atomic vapor cell, an atomic beam, or an atom trap. Atomic samples can contain one or several atomic species used for optical pumping, such as an alkali metal or an ion such as mercury. Additionally, atomic samples can comprise artificial atomic species, for example self-assembled quantum dots, qubits, or isolated single-spin systems such as nitrogen vacancy defects in diamond.
One example of an atomic system is an alkali-metal vapor cell, such as is used in conventional atomic clocks and magnetometers. Alkali-metal vapor cells contain a few droplets of an alkali metal, such as potassium, rubidium, or cesium. A buffer gas, such as nitrogen, other noble gases, or a mixture thereof; is required to be filled inside the cell to match the spectral profile of the pumping light, suppress the radiation trapping, and diminish alkali-metal atoms diffusing to the cell wall. The vapor cell is heated up to above room temperature to produce sufficient alkali-metal vapor.
Optical pumping systems often use a radiation source to excite atomic resonances. Conventionally, an atomic clock or a magnetometer measures the frequency at the maximum response of an atomic resonance. A radiation source is required to generate the oscillation signal and excite the resonance. A precise clock ticking signal is therefore provided by the output of the radiation source. The atomic resonances of alkali-metal ground-state hyperfine sublevels are especially useful for atomic clocks and atomic magnetometers. The hyperfine resonance is excited by the radiation source through radio frequency (RF) fields, microwave fields, or modulated light (CPT: coherent population trapping method).
U.S. Pat. No. 7,323,941, hereby incorporated by reference in its entirety into this application, describes a self-modulated laser system at an alkali-metal atom hyperfine frequency. The self-modulated laser system uses a polarization gain medium, such as an electronically pumped semiconductor, for example, quantum well heterojunction edge-emitting laser diode (ELD). A polarization gain medium outputs light with linear polarization. Alternation of photon spin is used inside the laser cavity. The vapor cell is positioned where the laser beam has the maximum alternation of the light polarization. Beams are recombined so that they emerge as a single beam of alternating circular polarization. The transmission of light through the external cavity is measured with a photodiode to generate a clock signal.
FIG. 1A depicts the main components of a conventional vapor-cell clock. Vapor cell 10 contains an alkali-metal vapor, typically rubidium or cesium, and an inert buffer gas. Pumping light propagates through vapor cell 10 and is collected by photodetector 12. Horn 13 beams microwaves at the cell from frequency synthesizer 14.
Modulation of the radiation source in an optical pumping system is often used in order to lock the radiation source to an atomic resonance. For example, conventional clocks use frequency or phase modulation to lock a microwave frequency to the ground state 0-0 hyperfine transition of an alkali metal. FIG. 1B shows pumping light transmission for microwaves near the 0-0 resonance. Modulation of the microwaves produces modulation in the transmitted light. Phase sensitive detection with lock-in amplifier 15 of transmission modulation generates the error signal in FIG. 1C, which is roughly the derivative of transmission. Feedback with PID controller 16 locks the frequency of the microwave source to the error signal zero-crossing, corresponding to the 0-0 frequency. Similar feedback systems are used in other optical pumping systems to lock a radiation source to an atomic resonance.
The light shift refers to the shift in an atomic resonance due to optical pumping. The light shift is a result of the AC Stark effect, and it depends on both the frequency and intensity of the pumping source. See B. S. Mathur, H. Tang, and W. Happer, Physical Review 171, 11 (1968). The light shift has important consequences in optical pumping systems. For example, in atomic clocks the light shift leads to errors such as clock frequency offset, noise, and drift. It is desirable to suppress or eliminate the light shift in optical pumping systems.
Conventional vapor-cell clocks, such as the passive rubidium frequency standard, do not eliminate the light shift. Instead, conventional clocks mitigate the light shift through using a lamp as a pumping source. Lamp pumping sources provide very stable pumping intensity with a large frequency spread, which act together to reduce and stabilize the light shift. Even with the best lamp sources, conventional clocks still experience clock frequency errors due to the light shift.
Theoretically, laser pumped standards should perform better than lamp pumped standards. However, it has been found that comparable laser pumped standards perform worse than lamp pump standards. Laboratory laser pumped standards have reached the expected performance, though at the cost of greatly increased complexity. As a result, laser pumped standards are still not yet commercially available. See J. Camparo, Physics Today 60, 33 (2007) and J. Vanier and C. Mandache, Appl. Phys. B 87, 565 (2007). The light shift is one of the main reasons for the limited performance of laser pumped standards.
One direct method to eliminate the light shift is to extrapolate measurements to zero pumping intensity. While feasible, this method is not practical for frequency standards or other devices that require continuous measurement and light shift elimination. Additionally, in practice, other effects such as non-uniform pumping light can introduce nonlinearity into the light shift, which may prevent this method from working.
Additionally, there are semi-continuous methods to eliminate the light shift, which have a main disadvantage in that measurement is interrupted in order to remove the light shift. One example is pulsed optical pumping, where the pumping light is turned off during measurement, as described in J. Vanier and C. Mandache, Appl. Phys. B 87, 565 (2007). Another example is periodic pumping frequency correction through measurement of light shift induced asymmetry of the resonance lineshape, as described in M. Hashimoto and M. Ohtsu, IEEE Trans. Instrum. Meas. 39, 458 (1990).
The light shift is truly eliminated if the pumping source is locked to a zero-light-shift frequency, or a frequency that produces no light shift. One such method uses slight pumping source intensity modulation to generate feedback and lock the optical frequency of a laser pumping source to a zero-shift frequency. The disadvantages of this method are that it requires additional modulation and expensive components, and that it typically is slow. See V. Shah, V. Gerginov, P. D. D. Schwindt, S. Knappe, L. Hollberg, and J. Kitching, Appl. Phys. Lett. 89, 151124 (2006), and F. Gong, Y.-Y. Jau, and W. Happer, Phys. Rev. Lett. 100, 233002 (2008).
Alternate continuous elimination methods include using more complicated optical pumping schemes, as described in M. Zhu, IEEE Frequency Control Symposium, 1334 (2007), multiple lasers, as described in J. Deng, IEEE Trans. UFFC 48, 1657 (2001), or modulated pumping light that cancels light shift, as described in C. Affolderbach, C. Andreeva, S. Cartaleva, T. Karaulanov, G. Mileti, and D. Slavov, Appl. Phys. B 80, 841 (2005). Disadvantages of these methods are that they require multiple lasers or modulation schemes, and that they add expensive components and additional complexity to the system.
An alternative to elimination is to suppress the light shift by stabilizing the frequency of the pumping source. One such method involves modulating a laser pumping source in order to lock the pumping source frequency to an optical absorption feature of an atomic sample, which may or may not be the same atomic sample in the optical pumping system. The main disadvantage of such previous methods is that the pumping source frequency is not locked to the zero-shift frequency, which may drift over time. Therefore, even if the locked pumping source frequency is initially offset to match the zero-shift frequency, the two frequencies will likely drift apart over time. Other disadvantages of such previous methods are that they require additional modulation, that the locked pumping source frequency varies with different atomic samples, and that the locked pumping source frequency may drift.
It is desirable to provide an improved method and system for suppressing or eliminating light shift in an optical pumping system.